The present invention is directed to an apparatus and method for compression molding plastic closures having a peripheral skirt with an internal thread, and more particularly to an apparatus and method for stripping the closure from a male mold core by rotating the core so as to unthread the closure.
U.S. Pat. Nos. 5,554,327, 5,670,100 and 6,074,583, assigned to the assignee of the present application, disclose apparatuses and methods for compression molding plastic articles such as plastic closures. A plurality of male and female mold assemblies are mounted on a turret in opposed circumferential arrays. The assemblies are coupled to fixed cams for movement into engagement with each other to form a mold cavity and compression mold a plastic charge into a closure, and then to move the assemblies axially away from each other to open the mold cavity. As the male mold assembly is retracted from the female mold assembly, the molded closure is stripped from the male mold core by a stripper sleeve that pushes the closure axially off of the core. The internal thread or threads on the closure skirt are thus wiped across the outer face of the mold core.
Although the arrangements disclosed in the noted patents have enjoyed substantial commercial acceptance and success, further improvements remain desirable. In particular, although the apparatuses and methods disclosed in the noted patents work well with relatively flexible and resilient thermoplastic materials such as polypropylene, they do not work as well as desired with thermosetting materials or with relatively stiff thermoplastic materials such as PET. When employing such materials, the closure cannot be axially stripped from the male mold core by expanding the closure skirt and wiping the threads across the radial surface of the core. The closure skirts are not sufficiently resilient for stripping by this technique, and the threads can become damaged. It is therefore a general object of the present invention to provide an apparatus and method for compression molding plastic closures that include facility for rotating the male mold core with respect to the outer holding member(s) for stripping the closure by unthreading the male mold core from within the closure. Another and more specific object of the invention is to provide an apparatus and method of the described character suitable for implementation in a continuously rotating molding apparatus of the type disclosed in above-noted U.S. Pat. Nos. 5,554,327, 5,670,100 and 6,074,583.
Apparatus for compression molding plastic closures having a peripheral skirt with an internal thread (or threads) in accordance with a presently preferred embodiment of the invention includes a first mold assembly having a male mold core and a second mold assembly having a female mold cavity. At least one of the first and second mold assemblies is moved relative to the other to bring the male mold core into the female mold cavity for compression molding a closure, and then is moved to open the cavity such that the closure is retained on the male mold core. The male mold core is rotated relative to the first mold assembly to unthread the core from within the closure, and thereby strip the closure from the core without stretching the closure skirt over the core or wiping the internal thread over the external surface of the mold core.
In the preferred embodiment of the invention, the first mold assembly includes a first shaft mechanism carried by a housing for rotary and axial movement, with the male mold core being coupled to the end of the first shaft mechanism. The male mold core is rotated by rotating the first shaft mechanism relative to the housing. The first shaft mechanism is mounted between axially spaced brackets, and a ballscrew shaft is rotatably carried by the brackets parallel to the first shaft mechanism. Drive gears rotatably couple the first shaft mechanism to the ballscrew shaft in the preferred embodiment of the invention, and a ballnut actuator is coupled to the ballscrew shaft for controlling rotation of the ballscrew shaft. In the preferred embodiment of the invention, the ballnut actuator is coupled to a fixed cam for controlling position of the ballnut actuator and rotation of the ballscrew shaft with axial motion of the first mold assembly. The contour of the cam controls the rate of rotation of the ballscrew shaft, the first shaft mechanism and the male mold core as the first mold assembly rises. A stripper sleeve on the first mold assembly engages the skirt of the closure and holds the closure against rotation as the mold core is rotated and retracted from the closure.
In accordance with another aspect of the present invention, a method of compression molding plastic closures contemplates provision of a first mold assembly having a male mold core and a second mold assembly having a female mold cavity. At least one of the first and second mold assemblies is moved with respect to the other to bring the male mold core into the female mold cavity for compression molding a closure, and the at least one mold assembly is then retracted from the other such that the mold cavity is opened and the closure is retained on the male mold core. The closure is removed from the male mold core by rotating the male mold core with respect to the closure so as to unscrew the mold core from the closure. This is accomplished by mounting a ballscrew shaft on the first mold assembly parallel to a first shaft mechanism to which the male mold core is secured, rotatably coupling the ballscrew shaft to the first shaft mechanism and rotating the ballscrew shaft, preferably by means of a ballnut actuator. The ballnut actuator may be held in fixed position so that the ballscrew shaft, the first shaft mechanism and the male mold core are rotated in opposite directions as the male mold core is lowered and raised, or more preferably is coupled to a fixed cam for controlling rotation of the ballscrew shaft, first shaft mechanism and male mold core as a function of axial movement of the male mold core. In the preferred embodiment, a plurality of first and second mold assemblies are mounted on a rotating turret in such a way that each assembly pair executes a complete operating cycle in sequence with the remaining assembly pairs.